A zener zap diode has been widely used for One Time Programming (OTP) application in CMOS Processes. In its “unblown” state, the un-zapped (before fusing) zener zap diode has a zener characteristic which appears as an open circuit under reverse bias condition. Moreover, the diode can be made to approach a short circuit by forcing a large reverse current (fusing current ˜10 mA) for a short period. Therefore, the applied reverse current maybe sufficient to overcome the junction breakdown voltage and causes a current flow across the reverse-biased PN junction. After the fusing current is reached, the characteristic behaviour of the diode is like a resistive short circuit and the junction no longer acts like an open circuit. By sensing the electrical state of the device, we are able to determine if it is open or short to store the logical state “1” or “0” for Read Only Memory (ROM) circuits. Once shorted, the change is irreversible; this is an OTP device.
The following prior art documents are of general relevance to the invention:    1. Patent GB2382220: Polysilicon diode antifuse    2. Patent EP0789403: Zener zap diode and method of manufacturing the same    3. U.S. Pat. No. 5,973,380: Semiconductor junction antifuse circuit    4. Donald T. Comer, “Zener Zap Anti-Fuse Trim in VLSI Circuits”, VLSI Design 1996, Vol 5, No: 1, pp. 89-100.
The resistivity of silicon at room temperature depends on the doping level and impurity material. Here is the data for phosphorus and boron impurities at room temperature:
Doping levelPhosphorus dopedBoron doped resistivity(atoms cm−3)resistivity in Ohm · cmin Ohm · cm101444.513310154.5813.510160.5271.4510170.08650.19710180.02250.040810190.005440.0088110200.0008030.00125
High values of resistance are usually for device well-region dopings where net doping is of the order ˜1017 cm−3 and resistance is ˜1000 ohms/square for layer with a diffusion depth of 1 micrometer. Low values of resistance are seen for ˜1020 cm−3 with resistances at ˜100 ohms/square for layers with a diffusion depth of ˜0.1 micrometers. Generally degenerately doped regions are thought of as “low” resistances (for silicon), but they are still relatively large compared to more conductive materials such as a metal, eg aluminium has a resistivity of 2.8×10−6 ohm·cm and iron is about 1×10−5 ohm·cm
Degenerate doping and zener junctions will now be discussed. Doping for n+ or p+ is the well known abbreviation for highly doped semiconductor material, n-type or p-type respectively, which is degenerate in nature. This semiconductor behaves more like a metal due to the number of electrical carriers available in the solid. Doping levels are approximately ˜5×1018 atoms of impurity per cm3 or higher to make degenerately doped silicon. Intrinsic (undoped) silicon atomic density is approximately 5×1022 atoms per cm3. We achieve a p+ or n+ doping using a low energy implanted dose of ˜1×1015 per cm2 which is activated and diffused to a depth of ˜0.15 micrometers thus making material with doping density of ˜1×1020 atoms per cm3.
For lower doped diffusions in silicon (n or p-type) the doping levels are usually much smaller at ˜1×1014 atoms per cm3 up to ˜1×1018 atoms of per cm3 of doping. The lower doses give to n-well or p-well are generally ˜1×1012 atoms per cm2 with silicon diffused depths of ˜1 um which gives ˜1×1016 atoms of per cm3 of impurity doping.
Zener diodes are formed when two heavily doped semiconductors meet and form an electrical n+/p+ junction. The junction forms a depletion region between the n+ and p+ which is extremely thin. This is a region depleted of carriers. Carriers can quantum mechanically tunnel through the thin layer—a zener conduction mechanism, when a relatively small reverse bias is applied to the junction (approximately 4V). The critical feature of zener conduction which is used in antifuse diodes is the low reverse breakdown voltage. We want to be able to take the device into reverse bias breakdown at a relatively low voltage, so that the circuitry supplying the programming voltage is as simple as possible. High voltages are generally more challenging to handle on modern CMOS due to the thin gate oxide regions and lower circuit operating voltages compatible with battery operation. If we apply high voltages, greater than a few volts, to standard CMOS devices we can cause irreversible device degradation or component destruction.
The diode junction used more commonly is n/p junction where the doping is lower and hence the depletion region thickness is larger. In that case it is too thick to allow tunnelling and the conduction process proceeds by a carrier avalanche ionisation mechanism when the reverse voltage is applied to a sufficient value, usually about 10V or higher. At such high voltages the electric field in the depletion layer is high enough to accelerate any electrons injected into it to a higher energy, which can then cause further electron pairs to be created by ionisation interaction with the surrounding material. In that case the conduction is not uniform through the junction; there are “hot spots”. Current crowding through hot spots can cause localised junction damage at points of weakness. These voltages are also high enough to cause irreversible electrical damage to CMOS circuits—eg the gate oxide breakdown voltage can be less than 10V.
Avalanche breakdown is in contrast to the zener conduction mechanism which is more uniform through the junction, stable and can be engineered to occur at lower voltages.
Zener diodes which need to pass a lot of current need to have a large junction area. These are usually made vertically in the material. But the antifuse construction is made laterally, with the highly doped regions butted at the surface of the semiconductor. In that case the cross-sectional area of the junction perpendicular to the current flow is quite small. We create a narrow region too ˜1 micrometer wide and about 0.15 micrometer deep. When a small current of ˜5 mA is passed through the small zener region this causes heating and consequent irreversible damage to the junction zone. The diode properties are destroyed and it becomes conductive in both directions. It behaves like an ohmic resistor instead of a diode which would only conduct significantly in the forward direction. The antifuse is designed so that the diode is normally operated in reverse bias. It moves from a poorly conducting diode-like state to a conductive resistive state once the junction has been damaged by a high current passing through. These are known in the industry as “zener zap diodes”.